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Odd lens coatings

I know the "ruby" coated lenses are a gimmick. Looking through bins with this gives the view a greenish blue cast.

My Bushnell Powerview 16x50 have a greenish coating. Not the deep green multicoating, but more of a reflective mirror coating. Looking through these bins, gives a slight red cast but not as strong color cast as the ruby ones.

My Pentax UCF-M 8x21 compacts also have a green coating. The view has a slight red cast, but lighter than the above.

Coatings have very little to do with what color cast you may see through a binocular. Remember, coatings are applied to allow light to pass through the lens and reduce reflection of light. The amount of light rejected by a coating is on the order of 1/2% to 1/4% per surface, not nearly enough tto make any differenccee to the color cast of a binocular. That is not to say a ruby coating could not change the color cast of a binocular, however a ruby coating is not an antireflection coating, it is a light rejection coating and should be avoided at all costs.

Possible reason for color cast is the degree of lens correcction. An apochromatic lens is color corrected to 1/8000 of the focal lengths of the color range. An achromat is correccted to only 1/2000 and in fact lesser achromats may be corrected to 1/1800. Lenses can be manufactured to be balanced far more towards one end of he color spectrum than another. Some lenses give a blue appearance, some give a strong red appearance, opposite ends of the spectrum. Focusing can change the color only slightly, but never remove it completely, simply because the lens is not corrected well enough to eliminate all color.

The coatings "melted"(kind of peeled)off the lenses after I'd carelessly left them uncovered on the dash board of a hire car in Gran Canaria in blazing hot sunshine and temperatures of around 100 degrees F.

That's good information. It explains why I see a slight yellow tone to the view through my Nikon 7x35 AE and Oberwerk 15x70. It is most noticeable when the daytime sky is in the view. It is really not an issue to me, just something I noticed.

I have several binos that show a clear color difference with each objective lens. This is esp. noticeable on my Garrett 100/45's. One lens is clearly a red / magenta hue and the other lens is clearly a green / blue hue. Optically both barrels perform the same.